Google Photos's automatic face detection and grouping is one of the many reasons I love the service, but I wouldn't call it perfect. Back when it began rolling out, several years ago, it created some duplicates, likely because that person had grown/changed a lot between two periods of time. But the algorithms seem a lot more accurate now, and so Google appears to be rolling out a feature to suggest merging recognizable duplicates under the same person.
Head over to Google Photos on the web, open the People tab (under your albums), and you'll likely see a suggestion box like the one above for merging similar faces. You can approve, reject, or tell Google you're not really certain about it. Although the two thumbnails don't appear clickable, you can open each of them to see all the pictures associated with that person and verify that the two are indeed the same.
Manually merging faces was possible before, but you had to do it manually by looking through all your people, spotting the dupes, and then giving them the same contact name to trigger a merge. This new suggestion box doesn't require any work on your part beside approving or rejecting the merge.
We first got a tip about this suggestion message showing up several weeks ago, but we couldn't reproduce it. Now, both Artem and I see it on our Photos accounts on the web, so it was likely being tested with a small sample before and recently began rolling out to more users.